1. Field of the Invention
The invention relates to printing machines, and more particularly to an anilox inking unit for a rotary offset printing machine.
2. Description of the Related Art
In offset printing machines, the interplay between ink and damping solution is a delicate matter. On the one hand, a certain proportion of water in the ink is desired and is also necessary for printing such that excess ink rapidly runs off the printing plate, but on the other hand too much water may result in problems and even in failure of the ink transfer. Depending on the degree of emulsification of offset printing ink, there are offset-stable emulsions with a water content of up to 40%, where the water present is very finely distributed in the printing ink, and offset-unstable emulsions with a water content of greater than 40% where the water is present in relatively large drops or as free water.
In the case of conventional inking units, an excessively high water content in the ink (free water) is not such a problem, since the roll surface, which is large overall, provides the free water with the possibility of evaporating. The water is also emulsified into the ink in the splitting points, of which there are a large number. Free water also impairs the splitting behavior of the ink. The effects in the printing process are an increase in the ink density and the clogging of screen areas. In the case of the conventional, long inking unit, the printer has the possibility of exerting an influence on the printing result by adjusting the zone screws.
Short inking units, which include the anilox inking unit, have a low number of ink transport rolls from the source of the ink to the printing plate. The relatively small roll surface provides little opportunity for excess water to evaporate. In addition, short inking units do not have the ability to influence the inking by means of zone screws, that is, it is not possible to counteract the failure of the ink transfer. The state of the art currently adds fresh ink or additives to the ink in the short inking unit. A high degree of care in the water supply is demanded of the printing machine operator. He is always working between the scumming limit (i.e., he will print rejects in the event of the smallest downward deviation of the water values) and disturbing the transfer of ink as a result of overdamping. The low supply of water also results in an increased outlay on cleaning and personnel resulting from the increased build-up of paper dust on the rubber blanket. This in turn leads to stoppages and renewed start-up rejects.
According to EP 0 224 033 B1, dry air is blown onto the surface of the screen roll in an anilox short inking unit before the roll passes a chamber-type doctor or a feed roll for filling with fresh ink. Although excess water is evaporated before the source of the printing ink is reached, water which is pressed into the dimples downstream of the nip between the ink applicator roll and the screen roll cannot be removed. In the chamber-type doctor, the dimples are flushed and water gets into the ink.